How to Clean the Exterior of a Dirty House?

I've recently found a house that I would like to purchase. But when I look at the exterior, I just couldn't believe how dirty it is. The white paint is smeared with black water marks (see photo below).

I've lived in my current house for six years and never seen anything like it. And I have never washed the outside of my current house before.

What causes this kind of mark? And how do I clean it? Do I just paint over it?

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